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From: markspace <-@.>
Newsgroups: comp.lang.java.programmer
Subject: Re: Volatile happens before question
Date: Thu, 19 Jan 2012 21:16:24 -0800
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On 1/19/2012 4:14 PM, raphfrk@gmail.com wrote:
> On Jan 19, 9:31 pm, markspace<-@.>  wrote:
>> I'm pretty sure that's the exact opposite of what is allowed.  Remember
>> that AtomicInteger::increment() is documented to have the same semantics
>> as both reading and writing a volatile.  I.e., from the perspective of a
>> given execution order, the write from increment() is guaranteed to
>> happen first.  Writes and reads may not be reordered around it.  I don't
>> think write are allowed to be interleaved with other writes this way.
>
> However, that only applies to volatile read/writes themselves, I
> think.
>
> There has to be a happens-before chain from W-write to R-read, in the
> example.  Otherwise, you can't say they happen separately.


No, and this is where a lot of folks mess up.  For locks (monitors), 
volatile variables, and other syncrhonization actions, there's 
specifically a transitive relationship. ALL writes prior to a 
synchronization action happen-before all subsequent reads.

So if you do this:

   int a = 1;
   int b = 2;
   int volatile v = 3;

Then a, b, and v all synchronize "together" and code like this:

   x = v;
   y = b;
   z = a;

Reads correctly when the read of v happens after the write of v.  (Note 
I'm specifically ignoring the case where the read of v does NOT happen 
after the write of v.  I think everyone agrees all bets are off in that 
case.)

What you did was a little different.

   y = b;
   x = v;

This can't synchronize, because even when the read of v does come after 
the write of v, b is still read before that, so it's value is really 
free to be anything.